In recent medical field, electronic endoscopes are frequently used for diagnoses and treatments. The electronic endoscope has a probing portion that is inserted into a body cavity of a subject under inspection, and an imaging unit including a CCD or the like is incorporated in a distal end of the probing portion. The electronic endoscope is also connected to a light source unit, so that light from the light source unit is projected from the distal end of the probing portion to illuminate the inside of the body cavity. While the inside of the body cavity is being illuminated, subject tissues inside the body cavity are imaged by the imaging unit. Captured images are processed in various ways in a processor which is also connected to the electronic endoscope, and the processed images are displayed on a monitor.
The electronic endoscope thus visualizes the inside of the body cavity of the subject under inspection in real time fashion. The captured images of the interior of the body cavity not only show the whole subject tissues but also individual details of the subject tissues, including fine or capillary vessels, deep blood vessels, pit patterns or gland orifice structures, as well as tissue surface asperities like concavity and convexity. Observing the condition of the subject tissues as the whole and in detail allows making diagnoses as to whether there are any lesions like a tumor.
When the operator of the endoscope detects a lesion from the image of the body cavity, the operator will usually scan the periphery around the detected lesion to search for metastasis of this lesion. For this purpose, the operator moves the distal end of the probing portion up and down or turns the direction of the distal end inside the cavity. However, the initially-detected lesion tends to fade out of the monitor screen with the up-down movement of the distal end of the probing portion. In order to trace a target like a lesion after the endoscope lose sight of the target, JPA 2002-095625 suggests detecting feature points of the target from endoscopic images and tracing the target with reference to its feature points.
However, once the detected feature points have faded out of the screen, it can be hard to detect the same feature points exactly through pattern recognition. Especially when the feature points represent a polyp in a deep labyrinth area of subject tissues, pattern recognition cannot ensure exact detection of the identical feature point.